1. Field of the Invention
This invention relates to a process for making perchloroethylene and hydrogen chloride by thermal noncatalytic chlorination of hydrocarbons and/or their partially chlorinated derivatives in the presence of carbon tetrachloride. In particular, it relates to a process for the production of perchloroethylene using carbon tetrachloride as a reactive diluent under conditions which maximize consumption of unwanted carbon tetrachloride while minimizing the production of heavy ends, such as hexachlorobenzene and other tarry products.
2. Background of the Invention
When perchloroethylene is manufactured by a conventional process involving chlorination of hydrocarbons and/or their partially chlorinated derivatives, substantial amounts of carbon tetrachloride are also obtained. In addition, substantial quantities of undesirable highly chlorinated .products, hereinafter referred to as heavy ends, are formed such as hexachloroethane, hexachlorobutadiene and hexachlorobenzene. Carbon tetrachloride is thought to be among the halocarbons which cause destruction of the ozone layer and it has also been used as a feedstock in producing environmentally deleterious halogenated chlorofluorocarbons. Because of the undesirable environmental effects of carbon tetrachloride, regulations governing the production and use of carbon tetrachloride are expected to result in a marked decrease in carbon tetrachloride production over the next decade. The heavy ends resulting from conventional chlorination processes are also undesirable and their disposal by burning can result in emission of undesirable compounds into the atmosphere. On the other hand, the more benign chlorinated hydrocarbons, notably perchloroethylene, will likely remain in demand because of their many practical uses. Perchloroethylene especially, because it is ecologically acceptable, is in high demand both as a solvent and as a starting material for the production of other chemicals. The present invention addresses these problems by providing a process that consumes carbon tetrachloride and minimizes formation of heavy ends in the production of perchloroethylene.
Direct thermal chlorination of methane, ethane, propane, ethylene, propylene, or their partially chlorinated derivatives exemplifies a conventional process for perchloroethylene production. The chemical reactions for the chlorination of these hydrocarbons and/or their partially chlorinated derivatives are exothermic. They can therefore result in carbon formation or result in an explosion from a runaway reaction if the temperature of the reaction is not controlled. One method of controlling the temperature in the reaction zone is to add a coolant or diluent to the feed mixture. A diluent is defined as any material that is injected into the reactor in order to moderate or control the reactor temperature. The use of vaporized carbon tetrachloride as a diluent to control reactor temperature is disclosed, for example in U.S. Pat. Nos. 2,577,388 and No. 2,442,323. These patents also disclose recycling of reaction products and use of other variables to control the ratio of carbon tetrachloride to perchloroethylene in the product stream. Use of a liquid diluent made up of chlorinated aliphatic compound such as carbon tetrachloride, perchloroethylene, hexachloroethane, hexachlorobutadiene, and mixtures thereof, is disclosed in U.S. Pat. No. 2,857,438.